The present invention is directed to a closure for use in conjunction with open ended medical implants, such as bone screws, hooks and the like that are used in certain types of spinal surgery. The purpose for the closure is to capture a rod member within the head of the implant and to also lock the captured rod member in position relative to the implant so that the two do not move rotationally or axially with respect to one another.
Medical implants such as bone screws and hooks are used in many types of spinal surgery wherein a structure is built about and within the spine to provide support or strength to diseased, missing or damaged spinal elements of the patient. The implanted apparatus includes a number of different parts which vary with each patient and that are linked together to form a stable support system. For example, bone screws are typically threadably mounted in the bones and have heads that receive rods or similarly shaped connectors that join other bone screws or other elements of the system together. Bone screws of this type may have either closed heads wherein a rod is threaded into the head from the side or an open head wherein a rod is laid in a channel formed in the head. The later type of bone screws are referred to as open ended bone screws. Such open ended bone screws are favored in many types of spinal surgery, since a rod does not have to be threaded through the bone screw which is difficult to do in the tight space provided and because the spine curves making insertion of a rod that follows the spinal curvature very difficult. On the other hand, the open ended bone screws allow the rod to be laid or drawn into the open channel of the head which greatly simplifies installation in comparison to threading the rod through a head of the bone screw.
While open ended medical implants such as bone screws, hooks and the like are often easier to use in comparison to closed end implants of the same type, the open ended implants do have associated problems. In particular, in open headed implant systems it is extremely important that there be no slippage or relative movement between the connecting rods and the implants, such as bone screws, to which the rods are joined. The rods in this case are usually not linear, but are curved to follow the curvature of the spine. If the rod is allowed to slip either axially or rotate within the bone screw or other connector to which the rod is secured, at a minimum the effectiveness of the implant is decreased and it is possible that the patient could be severely injured. Consequently, it is extremely important to secure the rod to the bone screw or other connector so that no slippage occurs. In some of the prior art devices, open headed implants, which typically have a pair of spaced arms, have been internally threaded and have received a threaded plug between the arms, which plug is designed to abut against the rod under torque and both capture the rod in the head and lock the rod into place.
The plug of the prior art has an outer cylindrical surface that is threaded and is received in the threads of the arms. In order to do this the plug has a comparatively large diameter so that the plug extends out sideways relative to the bone screw or the like or the bone screw must be made wider than necessary. That is, the plug makes the overall implant wider than is necessary. This presents problems to the surgeon installing the system, as space is very limited along the rods in many situations and there is not enough room for all of the total system structure to attach to the rods or, if there is sufficient room for attachment of elements, then there may be insufficient room for the surgeon to manipulate the rod by use of benders to shape the rod to conform to the proper spinal curvature, as is required in many of the surgeries using these devices. It is also noted that other prior art devices have a ring that goes around the outside of the arms. Such a ring is very bulky, taking up a great amount of space along the rods to which the implant is joined. Consequently, it is very desirable to eliminate this side to side extension of the closure along the axis of the rod. That is, it is desirable that the implant and closure both have a low side to side profile.
Secondly, as has been mentioned above, the rods are typically bent throughout the length thereof, such that the rods follow the curvature of the spine. Because of this, the rods seat in the open head of the bone screws and other implants in such a manner that the rod has curvature associated with it, even within the head. When plugs of the prior art seat against the rod, typically the rod is bowed so that the rod engages the plug only at the radially outer edges thereof on both sides. The plug is then torqued against the rod to tighten it down against the seat in the bone screw channel, but it is quite difficult to tighten the rod sufficiently so that the portion of the rod within the head has no curvature after tightening. Subsequently, during use of the device, the rod may be flexed or bent by activity of the person, especially greater activity than normal, such as occurs in an accident or the like, and the remaining curvature of the rod flexes somewhat, thereby loosening the plug. When this occurs, the rod may slip axially or rotate relative to the implant and/or the plug may become loosened and work out of the implant. Both of these situations are very undesirable.
Therefore, there is a need to provide a closure for bone screws and other medical implants having an open head such that the closure has both a relatively thin profile along the axis of the rod or the like and also where the closure has a mechanism whereby the rod can be locked centrally opposite the seat to prevent the rod from becoming loosened should the rod flex during usage.
Further, it is sometimes necessary to remove the bone screw closures after being installed. Because of the high torque applied to the closure and because of the relative smallness of the parts, it is often difficult to acquire sufficient purchase to rotate and remove the closure. Therefore, a removable closure and tools for removing the closures are also desired.